The present invention relates to a method for monitoring a measurement system, wherein the measurement system has a transmitter for determining the position of a movable element. The invention also relates to a measurement system wherein the measurement system has a transmitter for determining the position of a movable element.
These measurement systems are used to determine a position of a movable element, such as a motor shaft of a drive that drives the machine shaft, particularly in the technical field of automation engineering, incremental measurement systems in particular. In an incremental measurement system a sensor detects a reference mark and, after it moves over the reference mark, it can determine an absolute position because the increments that have been moved over can be counted without any gaps by means of a counter after moving over the reference mark (zero position). The count of the increments which have been moved over in this case corresponds to a rough position, with a fine position then being determined by interpolation, for example as described in DE 27 29 697 A1. The rough position and the fine position together provide the absolute position.
Particularly in the case of machine tools, production machines and/or robots, measurement systems such as these are usually supplied during normal operation with electrical power from an open-loop and/or closed-loop control device that controls movable elements of the machine. In many such measurement systems, if the open-loop and/or closed-loop control device is switched off, the count for the increments which have been moved over is then lost, and/or increments which are moved over after it is switched off are no longer included in the count. Once the open-loop and/or closed-loop control device has been switched on again, the reference mark must be moved over again in measurement systems such as these, before the absolute position can be determined.
According to the prior art, however, this problem can be solved by providing a battery to supply power to the measurement system even after the open-loop and/or closed-loop control device has been switched off. One example of such measurement system is described in EP 0 362 407 A1. In this example, power is not supplied to all the electrical components of the transmitter. Only a part of the transmitter is supplied with electrical power and, to this extent, the transmitter is operated in an energy-saving mode rather than in the normal mode. Electrical power can be supplied only to a single RAM, for example, for storing the count. In that case, however, any movements of the movable element when the open-loop and/or closed-loop control device is switched off can then not be detected.
Alternatively, it is also possible for a light-emitting diode of an optical transmitter that is otherwise operated continuously to be operated only on a pulsed basis, and for the interpolation electronics not to be operated at all. This results in the fine position no longer being detected at all, or no longer being detected as well as it normally would be, but this is still sufficient to count the increments moved over. Such transmitters are further described in DE 35 86 937 T2 and EP 0 158 781 A1.
if the power supply is inadequate, in the event of failure of the battery this prior art has the disadvantage that the count of the elements that have been moved over can be deleted or changed. In particular, this can occur in the event of an excessively long failure of the external power supply that ensures the power supply for the transmitter during normal operation. Once the open-loop and/or closed-loop control device has been switched on again, and therefore after the external electrical supply voltage for normal operation of the transmitter has been restored, there is then no longer any guarantee that the position determined by the transmitter will actually match the actual position of the movable element, because, in the meantime, the auxiliary supply voltage provided by the battery will have fallen too far to supply adequate power to the important electrical components of the transmitter.
As another alternative, DE 20 2004 000 413 U1 discloses a transmitter in which an electromechanical snap-action mechanism serves as an independent energy source and non-volatile memory is used to store the count. The relatively greater complexity of this mechanism is disadvantageous, among other things.
It would therefore be desirable and advantageous to provide an improved system and method for identifying incorrect position determinations that are the result of a power failure in a transmitter's electrical power supply that obviates these prior art shortcomings.